On the one hand, for instance, when foreign movies are showed in a country, a caption is often superimposed on the screen. As a caption data transmission system of this type, in a video disk or usual television broadcast, a video signal is transmitted with a caption being previously superimposed on a video image.
On the other hand, for instance, in the CAPTAIN system, a caption can be transmitted as a character code or dot pattern.
Further, in CD-G, graphics can be recorded using a subcode, which can be utilized to record a caption on a CD.
In CD-G, on the one hand, as shown in FIG. 26, data for one frame consists a 1-byte subcode and 32-byte data. In the 32-byte data, six samples, each consisting of two bytes, are assigned to L and R channels, respectively. Thus, the total is 24 bytes. For this 24-byte audio data, an 8-byte error correction code is added to form data of 32 bytes in total.
On the other hand, the subcodes for 98 frames are collected to form one block. The subcodes for the first two frames of the subcodes for the 98 frames are sync patterns S0 and S1. Various subcode data can be recorded in the subcodes for the remaining 96 frames. However, data for a track search is already assigned to the data of the P-channel and Q-channel of the 1-byte subcode (the individual bits are represented by P to W). Thus, graphics data can be assigned to the remaining R-channel to W-channel of 6 bits. That is, the extent to which graphics data can effectively assigned is 6.times.96 bits.
Since the data for one block is transmitted at a frequency of 75 Hz, the amount of data transmitted for one frame is 75.times.98 Hz. Consequently, the transmission bit rate of a subcode is 7.35 k bytes/s.
FIG. 27 shows the transmission format for such graphics data. As shown in the same figure, the data for six bits of R-channel to W-channel forms one symbol, and the data for 96 symbols constructs a packet which consists of four packs. Each pack is made up of 24 symbols, symbols 0 to symbols 23. Mode information is assigned to three bits, R, S, and T, of symbol 0, and item information is assigned to three bits, U, V, and W, of symbol 0. The combination of the mode and item, the following modes are defined.
______________________________________ MODE ITEM ______________________________________ 000 000 0 mode 001 000 graphics mode 001 001 TV-graphics mode 111 000 user mode ______________________________________
Since an instruction is assigned to symbol 1, and the parities for the mode plus item and instruction are assigned to symbol 2 and symbol 3, respectively, the extent to which graphics data can be assigned is 16 symbols, symbol 4 to symbol 19. The parities for the 20 symbols from symbol 0 to symbol 19 are assigned to the four symbols from symbol 20 to symbol 23.
In CD-G, graphics data can thus be assigned as binary data to the extent of 6.times.12 pixels for each pack. The pack rate is 75.times.4=300 packs/s. Accordingly, if one character is assigned to the extent of 6.times.12 pixels, 300 characters can be transmitted per second.
Since one screen defined in CD-G is 288 horizontal pixels.times.192 lines, it takes 2.56 seconds to transmit the characters for this one screen, as shown by the following equation: EQU (288/6).times.(192/12)/300=2.56 (1)
To provide a hexadecimal representation in each pixel takes 10.24 seconds, a time four times that of the above, because it is required that a different pattern be transmitted four times for one character pattern.
On the one hand, of these conventional methods, a method for transmitting a caption superimposed on a video image, such as a method on a video disk or the usual television broadcast, has a problem in that the user cannot switch the caption on or off as necessary. In addition, there is a problem in that it is not possible to prepare captions for a number of different languages so that the user can choose a desired one.
On the other hand, the method in the CAPTAIN system or CD-G has a problem in that the caption can be switched on or off as necessary, but the resolution is not sufficient.
That is, in the CAPTAIN system, the region available for the display of one screen is 248 horizontal pixels.times.192 lines, but a component digital TV signal has a resolution of 720 horizontal pixels.times.480 lines, and it is thus clearly that it is not sufficient when compared with the above resolution.
Further, in CD-G, since data of only one bit can be made to correspond to each pixel, data is represented in a binary form, and there is a problem in that, for instance, an aliasing phenomenon in which the slanted line portions of a character are notched, or a flicker phenomenon becomes marked, which is unpleasant for the view.
Moreover, binary information may be converted to multivalued information, for instance, by a filter, but it requires a high-precision filter, which is expensive. Also, the use of such a filter would degrade the background image as well.
In addition, if a pixel is represented by a hexadecimal value in CD-G, there is a problem in that it requires a time about four times that taken for binary representation, as described above, and it is thus difficult to switch the display of a caption at high speed.
The present invention was accomplished in view of the foregoing, enabling a high-quality caption to be displayed as desired by the user. In addition, it is to enable the display of a caption to be switched at high speed without degrading the video image in the background.